An integrated circuit package generally comprises a substrate made of epoxy impregnated fiberglass material, an integrated circuit made of silicon, and an encasing material that surrounds delicate electrical elements to protect them from mechanical damage and environmental exposure. During the manufacture of an integrated circuit package, the integrated circuit portion is generally referred to as an integrated circuit die.
There are certain manufacturing processes for an integrated circuit package (e.g., transfer molding of an exposed integrated circuit die) that require the integrated circuit die to be clamped. In these types of processes it is highly desirable that the integrated circuit die be located accurately in three dimensions. It is also highly desirable that the accurate three dimensional location of the integrated circuit die be repeatable.
The alignment of an integrated circuit die on a substrate is normally considered to be acceptable if the integrated circuit die is placed in its intended position within a tolerance of approximately one hundred microns (100 μm).
The placement of an integrated circuit die on the surface of the substrate may be considered as a problem of aligning the integrated circuit die within a conventional three dimensional rectangular coordinate system comprising an X-axis, a Y-axis and a Z-axis. The X-axis and the Y-axis form a plane that is coincident with the surface of the substrate. The Z-axis is perpendicular to the plane formed by the X-axis and the Y-axis. The value of Z represents the vertical location of the integrated circuit die with respect to the plane surface of the substrate.
To be within the normally acceptable tolerance of one hundred microns (100 μm), the location of the integrated circuit die on the substrate with respect to the X-axis must be within plus or minus one hundred microns (100 μm) of the intended X position. Similarly, the location of the integrated circuit die on the substrate with respect to the Y-axis must be within plus or minus one hundred microns (100 μm) of the intended Y position. Lastly, the location of the integrated circuit die on the substrate with respect to the Z-axis is ideally within a range of plus or minus twenty five microns (25 μm) to plus or minus fifty microns (50 μm) of the intended Z position.
In addition, the angular alignment of an integrated circuit die on a substrate must be accurate. Assume that the correct angular placement of an integrated circuit die is with a first side parallel to the X-axis and with a second side parallel to the Y-axis. If the integrated circuit die is not correctly aligned in its angular position, then it will be in a rotated position with respect to its correct angular position.
Similarly, it is possible for an integrated circuit die to be angularly misaligned with respect to the vertical Z-axis. Assume that the correct angular placement of an integrated circuit die is for the bottom of the integrated circuit die to be parallel with the surface of the substrate. Then the plane of the bottom of the integrated circuit die is to be perpendicular to the Z-axis. If the bottom of the integrated circuit die is inclined or tilted with respect to the surface of the substrate, then the integrated circuit die will not be in its correct angular position with respect to the vertical Z-axis.
In some types of silicon sensor applications part of the silicon surface of the integrated circuit die is exposed and is not covered by a protective molding. In some types of silicon sensor applications the position of the integrated circuit die must be precisely located with respect to the X, Y, and Z axes in order to have an acceptable yield after the molding process has been performed. That is, in order to improve the yield in silicon sensor applications the tolerance of the location of the integrated circuit die on the substrate must be minimized. This requires minimizing the variations in the X, Y and Z locations of the integrated circuit die, minimizing the angular rotation of the integrated circuit die in the X-Y plane, and minimizing the angular tilt of the integrated circuit die with respect to the Z-axis.
Therefore, it is often necessary that the height of the surface of an integrated circuit die above a substrate be closely controlled, and that the surface of the integrated circuit die be located in a plane that is parallel to the surface of the substrate. The term “planar” is used to refer to the surface of the integrated circuit die when the surface of the integrated die is located in a plane that is parallel to the surface of the substrate.
As previously mentioned, there are some types of manufacturing applications during which the surface of the integrated circuit die must be clamped in order to create a molded integrated circuit package that has some portion of the integrated circuit die surface area free of molding compound. The requirement for controlling the height and planarity of the surface of an integrated circuit die is particularly critical in these types of applications.
FIG. 1 illustrates a cross sectional view of a prior art molding machine 100 showing a clamping mechanism 110 of the molding machine 100 clamped against an integrated circuit die 120. The integrated circuit die 120 is attached to a laminate substrate 125 with a layer of die attach adhesive material 130. An electrical lead 135 connects integrated circuit die 120 to a metal layer (not shown) on laminate substrate 125.
Clamping mechanism 110 comprises a spring 140 that engages a clamp 145. Clamp 145 comprises clamp extension 150 that seats against integrated circuit die 120 due to the force exerted by spring 140. Molding compound is injected through the ports in molding machine 100 to fill cavity 155 and cavity 160 within molding machine 100. When the molding compound is injected the seal provided by clamp extension 150 prevents any molding compound from entering cavity 165 above integrated circuit die 120.
FIG. 2 illustrates a cross sectional view of the molding machine 100 shown in FIG. 1 in which the molding machine 100 has been removed from the molded integrated circuit package. The exposed surface of integrated circuit die 120 is denoted with reference numeral 200.
Various types of clamping mechanisms have been designed to be used with this type of prior art molding process. For example, U.S. Pat. No. 5,800,841 and U.S. Pat. No. 5,987,338 provide a clamping mechanism to exclude molding compound from at least a portion of an area of the surface of an integrated circuit die. Various types of clamping techniques have been employed to cushion the contact forces between the clamp and the surface of the integrated circuit die. However, in order to work properly these clamping techniques normally require that the surface of the integrated circuit die be located within the molding machine within a narrow vertical range.
Some of the prior art techniques develop the proper clamping forces by using a spring arrangement within the clamping mechanism. A spring arrangement requires precise deflections to produce the proper clamping forces. As described below, in order for the spring deflections (and thus the quality of the clamping) to be closely controlled, the vertical position of the top surface of the integrated circuit die must be accurately located within the molding machine.
In other types of clamping techniques, no springs are used and the clamping mechanism is therefore unyielding. This type of clamping technique depends on other elements within the package subassembly to act as surrogate springs for compliance. The surrogate spring elements allow the vertical position and orientation of the integrated circuit die to be adjusted to match the position and orientation of the clamping mechanism.
For example, one prior art technique employs cutouts under the mold to allow limited flexure of the substrate to accommodate some error in the die height or some error in the die planarity. This technique has a number of problems, including complex and synergistic problems with the design of the cutouts in the substrate support, the design of the substrate, and the die attach placement tolerances. Another problem with this technique is that high local contact forces on the die are needed to overcome the flexural stiffness of the laminate substrate in order to rotate and deflect the die so that the die is positioned against and conforms to the rigid clamping surface of the mold.
In another prior art technique a compliant material is added to the top of the integrated circuit die as a thin bead around the clamp contact area to provide accommodation of the misalignment between the integrated circuit die and the clamp.
These prior art techniques usually require more control of the die height and of the die planarity since the surrogate compliant components provide even less vertical accommodation than mechanical springs.
FIG. 3 illustrates a cross sectional view of a clamping mechanism 110 of a molding machine (not shown in FIG. 3) clamped against a non-planar integrated circuit die 320 that is tilted with respect to substrate 125. Die 320 is attached to substrate 125 with die attach adhesive material 130. There are significant adverse consequences that may be caused by improper die height. There are also significant adverse consequences that may be caused by lack of die surface planarity.
First, excessive die height may cause excessive clamping forces to occur. In all designs where clamping against the surface of the die is used, die heights that are larger than the design heights will result in excessive compressive or crushing forces acting on the surface of the die. When the forces become sufficiently high, these forces will damage the brittle integrated circuit base material or will damage the delicate electronic circuitry embedded in the top layers of the die.
Second, non-planarity of the integrated circuit die may cause excessive clamping forces to occur. That is, excessive local clamping forces can be generated when the die surface is not planar. In this case the clamping force is not uniformly distributed. The clamping force is concentrated at the highest contact area 330 between the clamp extension 150 and the top surface of die 320.
Third, insufficient die height may cause a poor clamp seal. Alternatively, when the die surface is too low, the clamping mechanism cannot develop sufficient pressure to seal the critical area against intrusion of molding compound.
Fourth, non-planarity of the integrated circuit die may cause a poor clamp seal. Improper sealing by the clamping mechanism will also occur if the planarity of the die surface is poor. In this case the clamping force is not uniformly distributed and some areas of the clamp will not provide enough clamping pressure to seal the critical area against intrusion of the liquid molding compound under high pressure.
Therefore, it is evident that it is highly desirable to be able to provide a high degree of control of the die height and of the die planarity for an integrated circuit die within a molding machine.
In a standard prior art die attach process, the die attach adhesive is dispensed as a paste or as a liquid. The die attach machine picks a die from a staging area and places the die into the soft die attach material. The die height and die planarity at the conclusion of this process are determined by a number of variables related to the die attach paste. These variables include the volume of the dispensed die attach paste, the pattern of the dispensed die attach paste, and the rheological properties of the dispensed die attach paste. Numerous mechanical factors related to the die attach machine are also important. These factors include the stability and uniformity of the grip of the machine on the die (usually by vacuum), the location of the grip of the machine relative to the central moment of inertia of the die, and the insertion parameters, such as insertion speed and targeted depth of insertion. These factors influence the final position and orientation of the die.
The die height and die planarity are also influenced by distortions imposed on the assembly after the mechanical steps of die placement and insertion. Distortions can also arise from the curing of the polymer adhesive. Distortions can also arise as the composite structure (which includes the cured die attach material) cools down from elevated temperatures.
In one prior art approach to help control the die height and die planarity, particles are added to the die attach adhesive material. The particles are intended to act as spacers or stops against which the die is forced. The particles in the die attach adhesive material are used to control the final bond line thickness and uniformity in the die attach adhesive material. This, in turn, ultimately assists in controlling the die height and die planarity.
Control of the final die height and die planarity through the use of die attach adhesive material loaded with spacer-particles depends on the equipment and techniques employed during the die attach process. The spacer-particle method does not address the problem of movement of the die in the soft material after the force from the insertion tool is removed (e.g., asymmetric forces from surface tension of the paste, buoyant and hydrostatic forces from the paste). The spacer-particle method also does address the problem of distortions (bowing and warpage) that are due to curing and cooling of the various materials that comprise the assembly. These movements and distortions influence the final die height and final die planarity.
In addition, the particle size and particle distribution within the die attach adhesive material must be carefully controlled. This adds expense and complexity to the manufacture, storage and use of the die attach adhesive material.
There is therefore a need in the art for an improved system and method for controlling the die height and the die planarity of an integrated circuit die. There is also a need in the art for an integrated circuit chip having an integrated circuit die with a precisely controlled die height and die planarity.